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By Mark Trevelyan and Alan Baldwin

RIO DE JANEIRO (Reuters) - Michael Phelps picked up his 19th gold medal on a second night of world records in the Olympic pool, with Katie Ledecky cruising to women's 400 meters freestyle victory and Britain's Adam Peaty running away with the men's 100 breaststroke.

Sweden's Sarah Sjostrom extended Saturday's record spree in winning the 100 meters butterfly, and Peaty and Ledecky followed suit in the next two finals before the U.S. men's team, with Phelps swimming second, took the 4x100 freestyle relay.

Six swimming world records have now fallen in two days of competition, and the United States moved level with Australia on two gold medals each.

Phelps, the most decorated Olympian of all time, now has 19 golds, two silvers and two bronzes. For his relay team mates Caeleb Dressel and Ryan Held, it was their first Olympic medal.

London Olympics 100 freestyle champion Nathan Adrian swam the final leg for the Americans, touching first ahead of France and Australia.

"On the block I thought my heart was going to explode, I was so hyped, so excited," said Phelps, competing in his fifth Olympics.

Ledecky, the rising U.S. swimming sensation, shaved 1.91 seconds off her own world record on the way to the 400m gold, the first stage of a rare treble she hopes to complete along with the 200m and 800m.

"It's pure happiness," the 19-year-old told reporters.

"I wanted the first 200 to hurt as little as possible and I really felt like I could build into it and really explode that last 50," she added after leading the race from start to finish.

"WHERE IS EVERYBODY?"

Britain's Peaty set his second world record in consecutive days, and seemed surprised by the margin of his victory - 1.56 seconds - over defending champion Cameron van der Burgh of South Africa.

"I touched the wall and looked around and thought 'where is everybody?' Peaty told reporters.

The last British man to win an Olympic swimming gold was Adrian Moorhouse in the same event in 1988, six years before Peaty was born. On a big night for the British team, Jazz Carlin won a silver medal behind Ledecky.

In the butterfly, Sjostrom became the first Swedish woman to win an Olympic gold medal, with Canada's 16-year-old Penny Oleksiak taking silver. American Dana Vollmer, the 2012 champion, clinched bronze.

"The feeling is totally crazy. I didn't realize it was a world record," Sjostrom said.

"I knew I was the big favorite. I was under pressure, so I tried to focus on no disasters. Before the start I said to myself: 'It's just a pool. It's nothing. I know what to do.'"

In a reminder of the doping controversies that dogged the build-up to the Olympics, there were loud boos for Russian breaststroker Yulia Efimova and the men's relay team.

Efimova, who has served two doping suspensions, succeeded in an appeal last week against being banned from Rio.

She was one of a number of Russians who argued successfully that excluding them from the Olympics would be punishing them again for the same offense.

She qualified second for Monday's 100 breaststroke final, 0.02 seconds behind Lilly King of the United States.